1. Field of the Invention
The present invention relates to a process for manufacturing high-grade metal alloys used by the investment casting industry to manufacture critical parts utilized in the "hot stages" of aircraft jet engines as well as turbocharger components for internal combustion engines.
2. Description of the Prior Art
At present, alloys bearing aluminum and titanium in various concentrations are required in the manufacture of certain metal parts which must resist high temperatures and corrosion. Such alloys are employed, for example, in the fabrication of parts for aircraft gas turbine engines. The metallurgical requirements for metals used to construct such parts are so stringent that the metals are termed "superalloys". The definition adopted by the American Society for Metals for a "superalloy" is: "an alloy developed for very high temperature service where relatively high stresses are encountered and where oxidation resistance is frequently required". More titanium is employed where an alloy of greater strength is required, while more aluminum is employed where the resultant alloy is to be highly resistant to oxidation.
Certain components of turbocharger units are currently produced by investment casting. An ingot of the alloy is first manufactured by vacuum processing, such as vacuum-induction melting, and is supplied in ingot form to an investment caster. The ingot is then remelted and cast in a mold to form the desired parts.
The raw materials for the manufacture of superalloys are classified broadly as either vacuum-melting grade or air-melting grade. Vacuum-melting quality material is the highest grade and must be clean, certified free of extraneous elements not tolerated in superalloys, and identified according to specific alloy. Vacuum-melting grade metals are produced by a number of different processing techniques. These processes include vacuum-induction melting, vacuum-arc remelting, electroflux, electron beam melting, and other processes. To date, special processing has been necessary to produce the raw materials of vacuum-melting grade to meet the very stringent specifications for the production of superalloys for critical components in gas turbine engines, as well as many other parts requiring a high degree of service integrity.
In the process of vacuum-induction melting an electric coil surrounds a refractory crucible and electromotive forces are used to heat the metals of the alloy in the crucible. In vacuum-induction melting the quality of the alloy is dictated predominantly by the quality of the raw materials. That is, the raw materials from which the ingot is formed must be of far greater purity than with other types of metallurgical alloy formation since many impurities are not removed during the vacuum-induction melting process.
Air-melting grade raw materials may contain some oxide scale and some detrimental materials which can be removed in air melting. Air melting is used primarily for wrought alloys used for plate, sheet, bar tube, and forging stock or for producing master alloys for subsequent remelting by the vacuum processes. Air-melting grade materials have been recently produced by the process of argon oxygen decarburization. The argon oxygen decarburization (AOD) process utilizes a trunion mounted open mouthed vessel lined with magnesite-chrome or dolomite refractory brick. Oxygen and inert gas (argon or nitrogen) are injected through under-bath tuyeres located in the side wall of the vessel. Heat generation results from the exothermic reaction of the bath components, and no external heat source is employed or required. The molten metal is initially blown with a high ratio of oxygen to inert gas. As the carbon content of the molten material decreases, the ratio of oxygen to inert gas is lowered step-by-step in order to obtain the most favorable thermodynamic condition. The AOD process desulphurizes the molten metal to very low levels and also removes carbon with high efficiency. However, the process also results in the removal of aluminum and titanium. In the manufacture of turbocharger parts, aluminum is essential to render the alloy resistant to oxidation, while titanium is essential in producing a part of sufficient strength. Accordingly, it has heretofore been necessary to manufacture ingot for the production of turbocharger parts by a vacuum melting process, rather than by an AOD process.
The process of producing components from ingots formed by vacuum-induction melting is extremely expensive as compared with the AOD. The process of forming an ingot containing greater than about 0.1% aluminum and titanium must be carried out in a vacuum due to the reactive nature of these elements with air. It has theretofore been possible to form such alloys solely by vacuum-induction melting. Due to the high cost of raw materials, and due to the expense of the vacuum-induction melting process itself, the ingots containing aluminum and titanium which are used by investment casters to produce metal parts are very, very expensive.